方案一:
update student s, city c
set s.city_name = c.name
where s.city_code = c.code;
方案二: 采用子查询
update student s set city_name = (select name from city where code = s.city_code);
update student s, city c
set s.city_name = c.name
where s.city_code = c.code;
update student s set city_name = (select name from city where code = s.city_code);